I-compounds--endogenous DNA markers of nutritional status, ageing, tumour promotion and carcinogenesis.

Certain bulky DNA modifications increase with age in laboratory animals without known exposure to carcinogens and are known as indigenous or I-compounds. Most of the spots detected on TLC maps by the nuclease P1-enhanced bisphosphate version or the monophosphate version of the 32P-postlabelling assay fit this definition of I-compounds. Chromatographic profiles and levels of these nucleotides greatly depend on animal species, strain, tissue and gender, but are also influenced by the diet and by chemical exposures. Thus, both genetic and environmental factors contribute to the formation of I-compounds. It appears that they are derived from endogenous DNA-reactive compounds generated during normal nutrient metabolism and that regulatory mechanisms exist which control their levels in tissue DNA.